Thanks guys. Stefan:

I would guess that this is due to memory fragmentation.

Timothy:

Your code is doing a lot of extra work and creating a lot of temporaries. I'd clean it up before I did anything else.

You suggestions turned out to be two sides of the same problem since the data amounts in the task is too close to the ceiling of memory. The approach: "First make it work. Then optimize" apparently isn't suitable here. Thanks /Søren